1. Field of the Invention
The present invention relates to a hydrophobic material used in the construction of a culvert for improving the drainage of the culvert. The material is capable of keeping the water-penetrating property of the soil which is returned to the culvert-draining groove for a long time.
2. Prior Art
In the conventional construction of a culvert, a drain pipe, such as an earthen pipe or a perforated plastic pipe, is placed in a culvert-draining groove from which the soil has been removed, and then the soil is returned to the culvert-draining groove to cover the drain pipe.
In addition, in order to further improve the draining capacity, a method has also been adopted, in which a cover material, such as brushwood, chaff and gravel, is spread around the drain pipe to increase water-penetrating gaps in a circumference of the drain pipe.
However, clods of soil, which are replaced in the culvert-draining groove, are broken down within a short time to become very small soil particles and the resulting soil particles adhere to the water-penetrating gaps in the soil or to the water-penetrating gaps of the cover material and additionally to the water-collecting holes of the drain pipe to stop water from entering the drain pipe. Also, earth and sand are accumulated in the drain pipe to eliminate the draining effect of the culvert in many cases.
The present inventors have proposed in Japanese Patent Publication No. Sho 50-23528 and Japanese Patent Publication No Sho 52-45403 that the surface of the soil clods, which are replaced in the culvert-draining groove or soil existing in a circumference of the culvert, be treated with a soil-aggregating agent to prevent the breakage of the clods over time, thereby using the soil itself as a filter medium.
The above-described method has been effective in improving the drainage of a culvert and has achieved a tentative success. However, since the above-described method is a method in which the removed soil clods or the removed groove portion is directly sprayed with an aqueous solution of a soil-aggregating agent to treat the surface of the clods or the soil existing in the circumference of the culvert and impregnate it with the aqueous solution of soil-aggregating agent, various kinds of mechanical apparatus are required in the construction of the culvert and about 50 to 60 liters of the aqueous solution of the soil-aggregating agent must be sprayed to treat 1 m.sup.3 of soil. Furthermore, increased time and labor are required in the construction of the culvert.
Additionally, the soil in the weak ground zone requiring the construction of the culvert is wet, and thus the marks of aqueous solution sprayed can not be visually confirmed, making it difficult to inspect.
The present inventors have also proposed a method, in which at least one of the four sides surrounding a culvert-draining groove in a longitudinal direction is covered with a filter medium obtained by housing a soil-aggregating agent in a water-permeable filter medium or by putting the soil-aggregating agent between the water-permeable filter media, as described in Japanese Patent Publication No. Sho 55-17167, as a result of their various investigations aimed at the improvement of the above-described method.
Cloth, nonwoven fabric, urethane foam, coconut mat and the like have been used as the filter medium in this method, and the soil-aggregating agent is housed in a bag formed by this filter medium or is put between two pieces of this filter medium.
The culvert is constructed by placing the filter medium, in which the soil-aggregating agent is housed or put between, on at least one of the four sides surrounding the longitudinal direction of the culvert-draining groove and by returning the soil into the groove.
According to this method, the filter medium, in which the soil-aggregating agent is housed or put between, is merely placed on the culvert-draining groove and no special mechanical apparatus is required. Moreover, since an advantage exists in this method in that the placement of the filter medium can be visually confirmed, inspection is easily achieved, and this method has been used in the construction of many culverts.
The cloth, nonwoven fabric and urethane foam, which have been used as a filter medium and have small water-penetrating gaps, have been found to be suitable for the material for housing the soil-aggregating agent therein or for putting the soil-aggregating agent therebetween, but a problem has occurred in that since the water-penetrating gaps are small, when muddy water passes through the water-penetrating gaps, soil particles are gradually accumulated on the filter medium to form a non-water-penetrating layer.
On the contrary, when coconut mat and a net of a large mesh having large water-penetrating gaps are used as the filter medium, there is not the possibility that the soil particles will accumulate on the filter medium. However, a problem has occurred in that these filter media are insufficient for housing the soil-aggregating agent therein or for putting the soil-aggregating agent therebetween, and the soil-aggregating agent is apt to come off during construction.
In addition, in the case of a filter medium for housing the soil-aggregating agent therein or for putting the soil-aggregating agent therebetween, the water-penetrating property is further reduced in the initial period after construction when a highly viscous aqueous solution of a soil-aggregating agent is used, so that the filter medium hardly exhibits the water-penetrating property.
As described above, both methods, in which the aqueous solution of soil-aggregating agent is directly sprayed (hereinafter simply referred to as the spraying method), and in which the filter medium housing the soil-aggregating agent therein or containing the soil-aggregating agent therebetween is buried (hereinafter simply referred to as the filter method), have advantages and disadvantages. Despite its disadvantages, the filter method is superior to the two methods in constructability and in confirming the marks of construction, and thus is still attractive.
Thus, the present inventors have conducted research aimed at solving the problem incidental to the filter method, that is, the problem that the water-penetrating property of the filter medium can not be improved. As a result of these investigations, the present inventors have discovered that the soil particles accumulating in the small water-penetrating gaps of the filter medium to remarkably reduce the water-penetrating property are silt and clay having diameters of 0.074 mm or less. In other words, the present inventors have been the first to discover the construction of a filter medium capable of preventing silt and clay from accumulating on the filter portion and capable of easily adhering the soil-aggregating agent therebetween, and thus they have completed the hydrophobic material of the present invention for improving the drainage of a culvert, capable not only of obtaining the effects equivalent to those of the spraying method but also of keeping the water-penetrating property of the soil around the culvert for a long time.